# In a structure, an earthquake causes a force in itself or it causes an intertial response in the system?

My teacher and I argued about whether a seismic event causing a force in itself or being an inertial response of the system. As I read in Chopra and Paz, the effect of an earthquake can be interpreted either as a force or as an inerntial response, either way yielding in the same results. I think it can be seen as both and my teacher says its an inertial response and nothing else. So I told my teacher and the class that we both were correct, but my teacher argued (and threatened) that he's right and I'm wrong. I wanted to see other peoples opinion.

• Inertia force is the (internal) structural response to the (external) displacement of the earth due to seismic force.
– r13
Commented Jan 27, 2022 at 1:29
• What do "intertial" & "interntial" mean? Inertial or internal? Please clarify by editing the question.
– Fred
Commented Jan 27, 2022 at 2:04
• Why don't you show them Chopra and Paz?
– AJN
Commented Jan 27, 2022 at 2:49
• I corrected "intertial" for "inertial", my bad. Commented Jan 27, 2022 at 3:57
• It is still showing "intertial" in the subject line Commented Jan 27, 2022 at 22:57

Don't confuse reality with equivalent mathematical representations.

A classic way of thinking about dynamic systems is through D'Alembert's principle, which allows us to convert a dynamic system into a static system under inertial forces.

I'm no expert in dynamic systems and have never heard of Chopra and Paz (the advantages of living in a country in the middle of a tectonic plate: no need to worry about earthquakes!). But I wouldn't be surprised if they're basically just applying D'Alembert's principle, converting the earthquake into equivalent forces.

So, are you correct that "the effect of an earthquake can be interpreted either as a force or as an [inertial] response, either way yielding in the same results"? Yes, you are. But that doesn't mean that the earthquake itself can be thought of as a force or an inertial response with equal validity.

The earthquake is the earth literally moving under your building's feet. Your structure's inertia then causes it to behave a certain way, generating certain forces throughout.

You can absolutely model that inertial response as a force, but that doesn't take away from the fact that --- in reality --- you're dealing with an inertial response.

You're free to question "if they give the same result, then who cares?" The answer is "pedants", but that doesn't take away from the fact that they aren't wrong.

Earthquake shakes the earth which is supprting the foundation of the building.

So it drags the foundation back and forth with it.

The building on top of the foundation has a lot of mass, inertia, so it remains were it is while the foundation is running away from under it.

Until the moment and shear generated in the deformed columns and shear walls of the first floor (or first few floors) causes the building to follow the foundation.

But the earth suddenly reverses its movement and drags the foundation back with it.

Now the building that hesitantly accelerated is not going to come back due to its inertia. the building keeps going in the wrong direction until the bending moment generated in deformed coumns forces it back.

This whiplash vibration will propogate true the entire height of the building and will shake the entire building until the building hopefully can redistribute the high magnitude jerks and their tribitury high stresses into a prefesigned acceptable mode of vibration and tolerable stresse.